Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces having differing light reflecting characteristics. The pattern of the bars and spaces encode information. In certain bar codes, there is a single row of bars and spaces, typically of varying widths. Such bar codes are referred to as one dimensional (1D) bar codes. Other bar codes include multiple rows of bars and spaces, each row typically having the same width. Such bar codes are referred to as two dimensional (2D) bar codes and examples include PDF417 and DataMatrix bar code formats. By virtue of the use of multiple rows of features (bars and spaces), 2D bar codes typically allow more encoded information than a 1D bar code in a given bar code area. Devices that read and decode one and two dimensional bar codes utilizing imaging systems that image and decode imaged bar codes are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. An illumination system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. Thus, an image of a field of view of the focusing lens is focused on the pixel array. Periodically, the pixels of the array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and attempts to decode the imaged bar code.
A continuing trend in bar code technology is increasing density of bar codes, that is, encoding more information in a given bar code area. For many items, the available area where a bar code may be imprinted is limited. Thus, if more information is desired to be encoded in a bar code, bar code density must be increased, hence the trend from 1D to 2D bar codes. Increasing the density of a 2D bar code requires increasing the number of bar code features (bars or stripes) in a given area. However, deceasing feature size leads to problems in imaging resolution and decoding of high density 2D bar codes, such at PDF 417 and DataMatrix bar codes which utilize very small feature size.
What is desired is an imaging-based bar code reader that provides for effective reading of high density 2D bar codes.